1. Field of the Invention
This invention relates to a method of forming a mixture gas in a direct injection type internal combustion engine, and a device for practicing the method.
2. Description of the Prior Art
A so-called "M-combustion system" has been known as a combustion system for a direct injection type diesel engine. In the system, fuel spray jetted by a fuel injection nozzle is positively caused to stick onto the wall of the combustion chamber, so that it is evaporated by the heat of the wall so as to form a mixture gas. In this case, the relationships between the time and the wall temperature which are required for evaporating the fuel on the wall are as indicated in FIG. 1. As is apparent from FIG. 1, the range of temperatures in which fuel is evaporated in a short time conforming to the combustion time (below 10 msec) in the engine is around 320.degree. C. On the other hand, under the ordinary operating conditions, the surface temperature of the piston is 200.degree. to 250.degree. C. at most. Therefore, it can be estimated that a period of time required for evaporating the fuel by the heat of the wall of the combustion chamber is relatively long.
Accordingly, in the M-combustion system, which is a typical example of the conventional combustion system, at the start of the engine or during the low speed operation of the engine the mixture gas is not satisfactorily formed. As a result, the combustion is unsatisfactory, the engine output is low, and the efficiency is also low. Furthermore, a lot of harmful exhaust gas containing for instance black smoke and HC is generated.
On the other hand, the recent tendency of an internal combustion engine for automobile is that its size is decreased (to decrease the quantity of exhaust gas) and the speed is increased. Accordingly, it has been required to improve the fuel injection system. That is, in order to maintain the best performance of the engine at all times under a wide range of operating conditions such as various engine speed and load conditions, it is essential to use a fuel injection system which can satisfactorily operate under all the operating conditions.
For instance, a fuel injection system for diesel engine essentially comprises an injection pump, an injection pipe, and a fuel injection nozzle. It is well known in the art that the spray characteristic of the fuel injection nozzle directly affects the performance of the engine.
For instance, in a conventional direct injection type internal combustion engine, the fuel injection nozzle is arranged substantially at the center of a recess which is formed in the top of the piston, so that a plurality of fuel sprays are jetted from the plurality of injection holes of the nozzle, respectively. The intake air swirl which is formed by the suction valve and the intake passage in the stroke of suction still remains at the end of the stroke of compression, thus flowing the fuel sprays in the direction of the swirl to form a mixture gas. The diameter of the aforementioned recess is, in general, 40 to 70% of the diameter of the piston or cylinder. Accordingly, in a small engine in which the diameter of the piston is below 100 mm, the diameter of the recess is necessarily small. If it is required to increase the ratio of compression, the diameter of the recess is further decreased. Accordingly, the fuel sprays jetted radially from the plurality of injection holes of the nozzle strike against the wall of the recess, thus remaining as liquid-state films or large droplets on the wall. Therefore, the fuel sprays thus jetted are not effectively burnt. As a result, the amount of mixture gas effective for combustion is decreased, the engine output is decreased, the fuel consumption is increased, and harmful smoke is generated.
A swirl injection nozzle, which is one of the fuel injection nozzles proposed by the inventors, has been applied to a direct injection type internal combustion engine, to confirm its usefulness. In this connection, it has been found that, in order to obtain the best performance of the engine by forming a desired mixture gas under a wide range of operating conditions, it is necessary to change the spray characteristic of the injection nozzle in conformance to the operating conditions. That is, it is necessary to develop swirl injection nozzles whose spray characteristics are determined according to the operating conditions.
There are available three different intermittent type swirl injection nozzles A, B and C. In the swirl injection nozzle A, as shown in FIG. 14, a tangential passage, namely a tangential groove 104 is formed in the outer wall of the needle valve 101. In the swirl injection nozzle B, as shown in FIG. 15, tangential ports 106 are communicated tangentially with a swirl chamber 105. In the swirl injection nozzle C, as shown in FIG. 16, a cylindrical partition member 109 is inserted into the nozzle body 107 in such a manner that it is in contact with the needle valve 101 and the inner wall 108 of the nozzle body 107, and tangential grooves 100 are formed in the outer wall of the partition member 109.
In each of the swirl injection nozzles A, B and C, the fuel is swirled by the tangential grooves or ports, and is atomized into fine droplets when jetted from the injection hole, thus forming a fuel spray. These injection nozzles A, B and C are large in spray angle and excellent in atomizing characteristic, and accordingly short in spray travel distance (or small in spray penetration), when compared with other fuel injection nozzles such as hole injection nozzles and throttle injection nozzles.
The conventioanl swirl injection nozzle has too large a spray angle as described above. Therefore, although the spray will not strike against the wall of the combustion chamber, the fuel droplets are held at rest in the combustion chamber.
If the fuel droplets are held at rest in the combustion chamber as described above, then they are surrounded by the combustion gas, as a result of which the combustion will not progress. That is, it is necessary that the fuel drops can proceed in the combustion chamber until the combustion is ended. Accordingly, a swirl injection nozzle for a direct injection type internal combustion engine should be one which can spray fuel satisfactorily in conformance with the operating conditions of the internal combustion engine. However, a method of designing or manufacturing such a swirl injection nozzle has not been proposed yet.
In each of the above-described intermittent type swirl injection nozzles A, B and C, the needle valve is slidably fitted in the valve hole formed in the valve body. Therefore, there must be a predetermined gap between the needle valve and the wall of the valve hole. Heretofore, in order to prevent the leakage of high pressure fuel from the gap, the gap is set to an extremely small value, about 2 to 5 .mu.m, or a swirl injection nozzle designing or manufacturing method utilizing a hydrodynamic means is employed. However, it is difficult to form the gap with high and uniform accuracy.
The spray angle of each of the intermittent type swirl injection nozzles A, B and C is large, as was described before. Therefore, the fuel droplets may stick onto the wall of the combustion chamber in the internal combustion engine. In order to prevent this difficulty, the spray angle should be set to a value which is suitable for the configuration or dimension of the combustion chamber. However, the spray angle of the conventional intermittent type swirl injection nozzle cannot readily be changed to a desired value.
Where a fuel resistance is represented by a flow rate coefficient C, and a spray configuration by a spray angle .alpha., the characteristic of the swirl injection nozzle A, B or C is affected by the factors of the tangential passage in the nozzle. It has been found that the diameter of the injection hole, the area of the tangential passage, the angle between the center line of the tangential passages and the central axis of the needle valve, and the gap between the tangential passage of the needle valve and the wall of the valve hole greatly affect the characteristic of the swirl injection nozzle. The effects of the area of the tangential passage and the gap between the tangential passage of the needle valve and the wall of the valve hole are as indicated in FIG. 17. As is apparent from FIG. 17, of these factors the aforementioned gap greatly affects the spray angle.
In this connection, the inventors have performed several experiments and analyses to improve the gap, the spray angle and the injection hole of the conventional swirl injection nozzle, thereby to obtain a spray characteristic which is in conformance with the operating condition of the internal combustion engine.
Heretofore, the gap is designed merely to prevent the leakage of fuel. On the other hand, the inventors has considered that the gap should be designed as an essential factor to determine the performance of the swirl injection nozzle. That is, in order to allow the swirl injection nozzle to have the most desirable characteristic factors, such as the aforementioned spray angle, flow rate coefficient and travel distance, which are suitable for the operating conditions of the internal combustion engine, the inventors have established an epoch making method which effectively utilizes the aforementioned gap, and proposes an intermittent type swirl injection nozzle as a mixture gas forming device, which can perform novel functions.